Why are some people skinny, even though they eat large amounts of food, while others become fat? Jeffery Gordon of Washington University in St. Louis thinks it’s because some people have types of bacteria that cause them to absorb more calories from their food.

You have two absorption systems in your body. You absorb most of your food as it passes through your small intestines. Food that is not absorbed in the small intestine goes to your colon. The colon contains a huge colony of bacteria that work to ferment undigested carbohydrates such as soluble fiber into short chain fatty acids and simple sugars that can then be absorbed through the colon walls into the bloodstream. Most people get about ten percent of their total calories from food absorbed through their colons.

Animal studies lead us to the next step. The dominant bacteria in the gut of obese mice are Firmicutes, types of bacteria that have more genes for breaking down the complex starches and fiber. Mice who are thin have more Bacteroidetes in their guts, and these bacteria are not as efficient in breaking down fiber and complex carbohydrates. Transplanting Firmicutes bacteria into the guts of lean mice made them fat.

These researchers also found that fat humans had far more Firmicutes bacteria than thinner ones. They then asked their overweight subjects to go on a low-fat, low-refined- carbohydrate diet for one year. As they lost weight, their bacteria changed to predominantly Bacteroidetes.

Today you may be able to lose weight by changing the composition of your diet in a way that changes the bacteria in your gut so you absorb fewer calories. In the future, you may be able to get a pill that contain primarily Bacteroidetes bacteria, take it daily and watch the pounds melt off because of the change in intestinal bacteria.

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Dear Dr. Mirkin: Which is more important for a long life, weight loss or exercise?

Being in shape helps to prolong your life, even if you are overweight and even if you store fat primarily in your belly, which is a major risk factor for diabetes, heart attacks, and probably certain types of cancers (JAMA, December 5, 2007). As people age, most gain weight and become progressively less active. Researchers at the University of South Carolina in Columbia showed that a person’s fitness level was a far stronger predictor of premature death than body fat. Those who were fit suffered less than half the death rate of those who were out of shape. They also showed that those who store fat primarily on their bellies are at significantly greater risk for dying early and that people with abdominal obesity who exercise are far less likely to die early than those who did not exercise.

The authors describe fitness as walking briskly at least five days a week. Abdominal obesity is defined as having a waist circumference over 40 inches for men or 35 inches for women. In this study, the least fit 20 percent had a death rate twice as high as people who did 30 minutes of walking five days a week. Those who were more fit had an even lower death rate. Furthermore, higher levels of fitness were inversely related to all causes of death in both the normal-weight and overweight groups.

Dear Dr. Mirkin: Is it true that you are more likely to have a heart attack when you have a cold?

Having an upper respiratory infection more than doubles your chances of getting a stroke or heart attack in the next week and more than triples your chances in the next three days, according to a study that appeared in the European Heart Journal (December, 2007).

Nobody really knows why, but inflammation is the leading theory on the cause of heart attacks and strokes. Your immunity is good for you because it helps protect you from infections However, if your immunity stays active, it can attack your body to cause inflammation which damages blood vessels and sets you up for clots that cause heart attacks and strokes.

This study does not mean that you should panic every time you get a cold. Tripling your chance of a heart attack still leaves you with a very low chance of suffering a heart attack or stroke. However, it does show the importance of getting flu immunizations, avoiding crowds during flu epidemics, and treating chronic infections.

Sports medicine doctor, fitness guru and long-time radio host Gabe Mirkin, M.D., and his wife, nutritionist Diana Mirkin bring you news and tips for your healthful lifestyle. A practicing physician for 50 years more

The Good Food Book

FREE! The Good Food Book: Live Better and Longer with Nature’s Best Foods, by Dr. Gabe Mirkin. With 100 of Diana Mirkin’s delicious, healthful recipes and lists of foods you can eat anytime.